1. Technical Field
The present invention relates to a piezoelectric transformer and a power supply including the piezoelectric transformer.
2. Background Art
As is known in the art, the piezoelectric transformer uses a piezoelectric vibrator. The piezoelectric vibrator has at least one pair of drive electrodes formed on and/or in a piezoelectric-ceramic plate. When an AC voltage is applied to the pair of drive electrodes, the plate vibrates due to the piezoelectric effect with a resonant mode such as a single wavelength resonant mode. When the plate is vibrating, variation of voltage potential is generated at any position of the plate. The variable voltage potential can be taken out from an output electrode formed on the position. A pair of output electrodes can be formed at different positions on the plate. Thus, the AC voltage applied to the drive electrodes as primary electrodes is transformed and is obtained at the output electrodes as secondary electrodes.
The piezoelectric transformer is used in a power supply in a circuit, for example, a lighting circuit of a back light for a liquid crystal display, a charging circuit for toner in a copying machine, or others. In the use, the piezoelectric transformer is mounted on and connected to a printed circuit board having other circuit components for the power supply.
Since it is important for the piezoelectric transformer that the piezoelectric plate vibrates, it is a serious problem to support the transformer without suppression of the vibration. A waveform of the vibration has vibration nodes and antinodes. Therefore, the transformer is supported at positions corresponding to the vibration nodes.
On the other hand, leads must be required to connect the transformer to the printed circuit board so as to apply the input voltage to the primary electrodes and take out the transformed voltage from the secondary electrodes. Usually, lead wires are connected to the primary electrodes and the secondary electrodes. However, the lead connection points are not always at positions corresponding to the vibration antinodes. The lead wires should be free without tension so as to interfere the vibration of the piezoelectric plate. The lead wires are troublesome for assembling, maintenance and other deal of an electrical circuit device such as the power supply, especially, a small size device.
Further, the primary and secondary electrodes are formed on different surfaces of the piezoelectric plate. Therefore, mounting and wiring operation of the transformer on a circuit board is complex. This results in large space for mounting the transformer on the circuit board and also in a high cost and large size of the circuit device.
Moreover, it is desired that the power supply has any safety provision because the piezoelectric transformer generates a high voltage.
Therefore, it is an object of the present invention to provide the piezoelectric transformer which has a simple structure of connection with an external circuit and is small sized.
It is another object of the present invention to provide a power supply including the piezoelectric transformer which is safe from the high voltage produced.